Chapter 6

1. Chapter 6 The TerrestrialPlanets

2. Outline: The four stages of planetary Development. The solid Earth : Interior, Magnetic Field, and earth’s active crust. The atmosphere : its origin Human effects on earth’s atmosphere.

3. Four stages of planets’ formation 3. Floating Stage: The planet was flooded by molten rocks and later by water which filled the lowlands. 4. Surface evolution: continues due to geological process and erosion.

4. The Earth’s Interior The density of the entire earth is 5.52 g/cm 3• The crust is much less dense, and the interior is made up of very heavy elements. • The heat in the interior of the earth may be: • left over from the formation of the planet. • Decay of radioactive material.

5. Earth had already passed through its 4 stages: 1. Differentiation - the separation of material according to density. - For Earth, the heavy iron and nickel settles in the center, producing a dense metallic core. Silicates have went outward to form a thin, fragile crust. It is from the densest (core), less dense (mantle) to the low-densest (crust), where the densest materials were able to sink to the core because Earth's interior was melted. - This differentiation had occurred due to the melting of Earth's interior caused by heat from the combination of radioactive decay plus energy released by in-falling matter during the planet's formation. Once the interior of Earth melted, the densest materials sank to the core.

6. 2. Cratering - when the crust formed • - Earth was heavily bombarded by craters in the debris-filled early solar system. • 3. Flooding - the flooding of the crater basins by lava, water, or both. • 4. Slow Surface Evolution • Terrestrial planets pass through these stages. • However, differences in masses, temperature, and composition emphasize some stages over others—producing surprisingly different worlds.

7. The comparison of one planet with another is called comparative planetology. It is one of the best ways to analyze the worlds in our solar system. You will learn much more by comparing planets than you could by studying them individually. In this chapter, you will visit five Earthlike worlds. This preliminary section will be your guide to important features and comparisons.

8. You are about to visit Earth, Earth’s moon, Mercury, Venus, and Mars. It may surprise you that the moon is on your itinerary. After all, it is just a natural satellite orbiting Earth and isn’t one of the planets. Five Worlds

9. Five Worlds • The figure compares the five worlds you are about to study.

10. The terrestrial planets Mercury, Venus, Earth and Mars in true colors, sizes to scale

11. Five Worlds • The first feature to notice is diameter. The moon is small. Mercury is not much bigger. Earth and Venus are large and similar in size to each other. Mars, though, is a medium-sized world.

12. You will discover that size is a critical factor in determining a world’s personality. Small worlds tend to be internally cold and geologically dead. However, larger worlds can be geologically active. Five Worlds

13. Formation of planets • Earth formed 4.6 billion years ago from the inner solar nebula. • Four stages of planets’ formation:1. Differentiation Stage: divided into layers High-density metal cores, thick mantle of dense rocks. Rocky, low-density crusts. 2. Cratering Stage: The planet is heavily bombarded by leftover planetesimals and fragments.

14. All terrestrial planets have approximately the same type of structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but has a much smaller iron core. Io and Europa are other satellites that have internal structure similar to terrestrial planets. Terrestrial planets can have canyons, craters, mountains, volcanoes, and other surface structures. Terrestrial planets possess secondary atmospheres, generated through internal volcanism or comet impacts, in contrast to the gas giants, whose atmospheres are primary, captured directly from the original solar nebula

15. The terrestrial worlds are made up of rock and metal. They are all differentiated: Rocky, low-density crusts High-density metal cores Mantles composed of dense rock between the cores and crusts Core, Mantle, and Crust

16. Core, Mantle, and Crust • As you have learned, when the planets formed, their surfaces were subjected to heavy bombardment by leftover planetesimals and fragments. • The cratering rate then was as much as 10,000 times what it is at present. • You will see lots of craters on these worlds—especially on Mercury and the moon.

17. Notice that cratered surfaces are old. For example, if a lava flow covered up some cratered landscape to make a new surface after the end of the heavy bombardment, few craters could be formed afterward on that surface. This is because most of the debris in the solar system was gone. So, when you see a smooth plain on a planet, you can guess that the surface is younger than the cratered areas. Core, Mantle, and Crust

18. Core, Mantle, and Crust • One important way you can study a planet is by following the energy. • The heat in the interior of a planet may be left over from the formation of the planet. • It may also be heat generated by radioactive decay. • In any case, it must flow outward toward the cooler surface where it is radiated into space.

19. In flowing outward, the heat can cause phenomena such as: Convection currents in the mantle Magnetic fields Plate motions Quakes Faults Volcanism Mountain building Core, Mantle, and Crust

20. Heat flowing upward through the cooler crust makes a large world like Earth geologically active. In contrast, the moon and Mercury—both worlds—cooled fast. So, they have little heat flowing outward now and are relatively inactive. When you look at Mercury and the moon, you can see their craters, plains, and mountains. Core, Mantle, and Crust

21. You might ponder two questions. One, why do some worlds have atmospheres while others do not? You will discover that both size and temperature are important. Atmospheres

22. The second question is more complex. Where did these atmospheres come from? To answer the question, you will have to study the geological history of these worlds. Atmospheres

23. When you think about Earth’s atmosphere, you should consider three questions. How did it form? How has it evolved? How are we changing it? Earth’s Atmosphere

24. Also, the final stages of planet building may have seen Earth and other planets accreting planetesimals rich in volatile materials—such as water, ammonia, and carbon dioxide. Thus, the primary atmosphere must have been rich in carbon dioxide, nitrogen, and water vapor. The atmosphere you breathe today is a secondary atmosphere produced later in Earth’s history. Earth’s Atmosphere

25. Earth’s first atmosphere—its primary atmosphere—was once thought to contain gases from the solar nebula, such as hydrogen and methane. Modern studies, however, indicate that the planets formed hot. So, gases such as carbon dioxide, nitrogen, and water vapor would have been cooked out of (been outgassed from) the rock and metal. Earth’s Atmosphere

26. Also, the final stages of planet building may have seen Earth and other planets accreting planetesimals rich in volatile materials—such as water, ammonia, and carbon dioxide. Thus, the primary atmosphere must have been rich in carbon dioxide, nitrogen, and water vapor. The atmosphere you breathe today is a secondary atmosphere produced later in Earth’s history. Earth’s Atmosphere

27. Soon after Earth formed, it began to cool. Once it cooled enough, oceans began to form, and carbon dioxide began to dissolve in the water. Carbon dioxide is highly soluble in water—which explains the easy manufacture of carbonated beverages. As the oceans removed carbon dioxide from the atmosphere, it reacted with dissolved compounds in the ocean water—to form silicon dioxide, limestone, and other mineral sediments. Thus, the oceans transferred the carbon dioxide from the atmosphere to the seafloor and left air richer in other gases, especially nitrogen. Earth’s Atmosphere

28. This removal of carbon dioxide is critical to Earth’s history. This is because an atmosphere rich in carbon dioxide can trap heat—by the greenhouse effect. When visible-wavelength sunlight shines through the glass roof of a greenhouse, it heats the interior. Infrared radiation from the warm interior can’t get out through the glass. Heat is trapped in the greenhouse. Earth’s Atmosphere

29. The temperature climbs until the glass itself grows warm enough to radiate heat away as fast as sunlight enters. Of course, a real greenhouse also retains its heat because the walls prevent the warm air from mixing with the cooler air outside. This is also called the ‘parked car effect’—for obvious reasons. Earth’s Atmosphere

30. Earth’s Atmosphere • Like the glass roof of a greenhouse, a planet’s atmosphere can allow sunlight to enter and warm the surface.

31. Carbon dioxide and other greenhouse gases such as water vapor and methane are opaque to infrared radiation. So, an atmosphere containing enough of these gases can trap heat and raise the temperature of a planet’s surface. Earth’s Atmosphere

32. It is a common misconception that the greenhouse effect is always bad. However, without the effect, Earth would be colder by at least 30 K (54 F). The planetwide average temperature would be far below freezing. The problem is that human civilization is adding greenhouse gases to those that are already in the atmosphere. Earth’s Atmosphere

33. For 4 billion years, Earth’s oceans and plant life have been absorbing carbon dioxide and burying it—in the form of carbonates such as limestone and in carbon-rich deposits of coal, oil, and natural gas. However, in the last century or so, human civilization has been: Digging up those fuels Burning them for energy Releasing the carbon back into the atmosphere as carbon dioxide Earth’s Atmosphere

34. Earth’s Atmosphere • This process is steadily increasing the carbon dioxide concentration in the atmosphere and warming Earth’s climate. • This is known as global warming.

35. Global warming is a critical issue. This is not just because it affects agriculture. It is also changing climate patterns that will warm some areas and cool other areas. In addition, the warming is melting what had been permanently frozen ices in the polar caps—causing sea levels to rise. A rise of just a few feet will flood major land areas. When you visit Venus, you will see a planet dominated by the greenhouse effect. Global Warming

36. When Earth was young, its atmosphere had no free oxygen. Oxygen is very reactive and quickly forms oxides in the soil. So, plant life is needed to keep a steady supply of oxygen in the atmosphere. Photosynthesis makes energy for the plant—by absorbing carbon dioxide and releasing free oxygen. Oxygen in Earth’s Atmosphere

37. Ocean plants began to manufacture oxygen faster than chemical reactions could remove it about 2 to 2.5 billion years ago. Atmospheric oxygen then increased rapidly. As there is oxygen in the atmosphere now, there is also a layer of ozone (O3) at altitudes of 15 to 30 km. Oxygen in Earth’s Atmosphere

38. Many people hold the common misconception that ozone is bad—because they hear it mentioned as part of smog. Indeed, breathing ozone is bad for you. However, the ozone layer is needed in the upper atmosphere. This layer protects you from harmful UV photons. Oxygen in Earth’s Atmosphere

39. However, certain compounds called chlorofluorocarbons (CFCs), used in refrigeration and industry, can destroy ozone when they leak into the atmosphere. Since the late 1970s, the ozone concentration has been falling. The intensity of harmful ultraviolet radiation at Earth’s surface has been increasing year by year. Oxygen in Earth’s Atmosphere

40. Note that ozone depletion is an Earth environmental issue that is separate from global warming. This poses an immediate problem for public health on Earth. However, it is also of interest astronomically. When you visit Mars, you will see the effects of an atmosphere without ozone. Oxygen in Earth’s Atmosphere

41. Atmospheres The atmospheres of at least the inner planets has evolved since they formed. This is clearest for the Earth. The Earth’s original atmosphere was probably similar to Venus in composition, consisting of carbon dioxide and nitrogen. The evolution of photosynthesis converted carbon dioxide in the Earth’s atmosphere to oxygen, increasing the amount of O2 in it from an initial 0.01% to its current 22% level.

42. Each of the planets has a different atmosphere, although there are clear similarities between the atmospheres of the four terrestrial planets and the four gas giant planets. The terrestrial planets are rich in heavier gases and gaseous compounds, such as carbon dioxide, nitrogen, oxygen, ozone, and argon. In contrast, the gas giant atmospheres are composed mostly of hydrogen and helium.

43. Here is basic information on the atmosphere of each planet. Mercury has a very thin, almost undetectable atmosphere composed of sodium and potassium gas. These elements were likely blown from the surface of Mercury by the solar wind. The atmosphere of Venus is composed mainly of carbon dioxide with minor amounts of nitrogen and trace amounts of nitrogen, helium, neon, and argon. The surface of Venus, though, is completely hidden by a cloudy atmosphere even thicker than Earth’s.

44. The Earth's atmosphere primarily composed of nitrogen and oxygen. Minor gases include and carbon dioxide, ozone, argon, and helium. Mars' atmosphere is a thin layer composed mainly of carbon dioxide. Nitrogen, argon, and small traces of oxygen and water vapor are also present.

45. Earth is an active planet. It has a molten interior and heat flowing outward to power volcanism, earthquakes, and an active crust. Almost 75 percent of its surface is covered by liquid water. The atmosphere contains a significant amount of oxygen. From what you know of the formation of Earth, you would expect it to have differentiated. Earth: Planet of Extremes

46. Earth’s mass divided by its volume gives you its average density—about 5.5 g/cm3. However, the density of Earth’s rocky crust is only about half that. Clearly, a large part of Earth’s interior must be made of material denser than rock. Earth’s Interior

47. Earth’s Interior • Each time an earthquake occurs, seismic waves travel through the interior and register on seismographs all over the world. • Analysis of these waves shows that Earth’s interior is divided into: • A metallic core • A dense rocky mantle • A thin, low-density crust

48. Earth’s Interior • The core has a density of 14 g/cm3,greater than lead. • Models indicate it is composed of iron and nickel at a temperature of roughly 6,000 K. • The core is as hot as the surface of the sun. • However, high pressure keeps the metal a solid near the middle of the core and a liquid in its outer parts.

49. Earth’s Interior • Two kinds of seismic waves show that the outer core is liquid. • P waves travel like sound waves, and they can penetrate a liquid. • S waves travel as a side-to-side vibration that can travel along the surface of a liquid but not through it. So, Earth scientists can deduce the size of the liquid core—by observing where S waves get through and where they don’t.

50. Earth’s magnetism gives you further clues about the core. The presence of a magnetic field is evidence that part of Earth’s core must be a liquid metal. Convection currents stir the molten liquid. As the liquid is a very good conductor of electricity and is rotating as Earth rotates, it generates a magnetic field through the dynamo effect. This is a different version of the process that creates the sun’s magnetic field. Earth’s Interior